Engines may be configured to deliver fuel to an engine cylinder using one or more of port and direct injection. Port fuel direct injection (PFDI) engines are capable of leveraging both fuel injection systems. For example, at high engine loads, fuel may be directly injected into an engine cylinder via a direct injector, thereby leveraging the charge cooling properties of the direct injection (DI). At lower engine loads and at engine starts, fuel may be injected into an intake port of the engine cylinder via a port fuel injector, reducing particulate matter emissions. During still other conditions, a portion of fuel may be delivered to the cylinder via the port injector while a remainder of the fuel is delivered to the cylinder via the direct injector.
During engine operation with direct injection enabled, fuel flow through the direct injector nozzle maintains the direct injector tip temperatures substantially lower (e.g., around 100° C.). In comparison, during periods of engine operation where direct injection is disabled and no fuel is being released by the direct injector (e.g., during conditions where only port injection of fuel is scheduled), the direct injector tip temperature may become substantially higher (e.g., around 260° C.). When fuel is subsequently injected from the direct injector, the fuel may be at the elevated temperature, and therefore at a lower density than expected, resulting in unintended fueling errors. For example, due to less fuel being delivered than intended, the direct injection can result in a lean air-fuel ratio error. In one example, when the injector temperature rises by 80° C., a 4% lean error is created.
One example approach for compensating for an elevated direct injector tip temperature is shown by VanDerWege et al. in U.S. Pat. No. 9,322,340. Therein, responsive to an elevated temperature of a knock control fluid at a time of release from a direct injector, a pulse width of the injection is adjusted. In particular, a longer direct injection pulse width is applied as the predicted temperature of the fuel at the time of release from the direct injector increases.
However the inventors herein have recognized potential issues with the above approach. As one example, even with the adjustment of '340, fueling errors may persist due to differences in the behavior of the fuel temperature and tip temperature over the duration of direct injector deactivation, as well as during the subsequent direct injection. For example, heat transfer to the direct injector over the period of deactivation may differ based on whether cylinder combustion continued via port injection, average cylinder load if cylinder combustion did continue, whether all cylinder combustion was stopped, whether air continued to be pumped through the cylinder when combustion was stopped due to selective fuel deactivation without valve deactivation, whether both the fuel injector and the valves were deactivated when combustion was stopped, whether the engine was still spinning when combustion was stopped, etc. Some of these factors may also have an effect on the fuel temperature, albeit different from the effect on the direct injector tip temperature. In still another example, when the direct injector is reactivated and fuel is released therefrom, the injector tip temperature may cool at a faster rate than the fuel temperature. As a result of these variation, if the direct injection of knock control fluid is corrected to compensate for the elevated temperature of the fuel at the time of release, the density change may be overestimated. The pulse width of the direct injection may be increased more than required (or longer than required), resulting in a rich air-fuel ratio error. Alternatively, the density change may be underestimated with the pulse width of the direct injection increased less than required (or shorter than required), resulting in a lean air-fuel ratio error. As yet another example, in the approach of '340, the fuel temperature is calculated based on an inferred fuel rail temperature. However, during engine transients, the fuel rail temperature may remain stable. This causes the calculated fuel temperature to be held substantially constant while the actual fuel temperature increases.
In one example, some of the above issues may be addressed by a method for an engine comprising: responsive to deactivation of a direct injector, estimating a direct injector tip temperature different from fuel temperature based on cylinder conditions including cylinder combustion conditions, cylinder valve operation, and port injector operation during the deactivation; and responsive to reactivation of the direct injector, adjusting a direct injection fuel pulse based on each of the estimated direct injector tip temperature and fuel temperature. In this way, direct injection fueling errors can be reduced.
As an example, an engine may be configured with both port and direct injection capabilities. During engine operation, including during cylinder combusting and cylinder non-combusting conditions, an engine controller may continuously estimate a direct injector tip temperature different from a fuel temperature. The fuel temperature may be estimated via a fuel rail temperature sensor. The direct injector tip temperature may be determined as a function of heat flow into the direct injector (such as due to combustion heat when cylinder combustion is enabled) as well as cooling flow into the direct injector (such as due to fuel being replenished at the injector). As such, the heat flow and cooling flow estimates may vary based on multiple combustion parameters such as whether the direct injector is activated or not, whether cylinder combustion via port injection is continuing or not when the direct injector is deactivated, whether cylinder valves are operating or not when the direct injector is deactivated and the cylinder is not combusting, average cylinder load when the direct injector is deactivated and the cylinder is combusting, duration of direct injector deactivation, etc. The controller may determine a steady-state direct injector tip temperature when direct injection is enabled and then monitor a transient change in the direct injector tip temperature while direct injection is disabled. As such, the fuel temperature may fluctuate less dramatically than the tip temperature. The controller may concurrently determine a fuel density correction factor based on the tip temperature relative to the fuel temperature, and apply the correction factor to a nominal fuel density estimate so that fluctuations in the fuel density can be monitored in real-time. At the time of reactivation of the direct injector, the controller may adjust a direct injection pulse-width based on the corrected fuel density estimate. For example, at a time of direct injector reactivation after a period of DI deactivation where cylinders continued to receive fuel from the port injectors and combust, the DI tip temperature may have risen above the steady-state temperature. Accordingly, the controller may compensate for a drop in fuel density by increasing the fuel pulse-width by a larger amount. In comparison, at a time of direct injector reactivation after a period of DI deactivation where cylinders did not combust but air continued to be pumped through the valves (e.g., a DFSO event), the DI tip temperature may have fallen below the steady-state temperature. Accordingly, the controller may compensate for a rise in fuel density by increasing the DI fuel pulse-width by a smaller amount, or by decreasing the DI fuel pulse-width. In addition, the pulse-width may be varied over a duration since the reactivation with a time constant that is based on the transient change in tip temperature.
In this way, fuel injection settings of a direct injector may be adjusted to compensate for changes in fuel density due to different degrees of heating of the fuel and the injector tip over a duration of direct injector disablement. The technical effect of compensating for the rate of change in fuel temperature differently from the rate of change in tip temperature is that the different temperature profiles may be accounted for when direct injection is re-enabled. By continuously estimating a direct injector tip temperature based on variations in heat flow and cooling flow to the injector, temperature-induced changes in fuel density can be more accurately estimated and an injection pulse-width can be appropriately adjusted without incurring (lean or rich) air-fuel ratio excursions. In addition, the charge cooling effect of the direct injected fuel can be better leveraged. Furthermore, direct injector fouling and thermal degradation can be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.